Implement an easy way to toggle between dark mode and light mode on the homepage

[tommcclean]

Is your feature request related to a problem? Please describe.
Most people who know the feature exists like the choice, but currently it is quite hidden (in the language switcher). So having something on the homepage to toggle between themes could help user's to learn about the feature, choose their preference and may help them to become returning users

Describe the solution you'd like
Some way (yet undecided) to toggle theme on the homepage. This should continue to set the cookie so that the theme is remembered.

Describe alternatives you've considered
An alternative was pulling the theme controls out of the language switcher and into a theme switcher in the header. This would be more effort though and may be difficult to make work with UI on a small mobile phone.

Additional context
You can change the theme of the website by clicking on the flag in the top right corner of the website, and selecting either dark or light theme. This preference is stored as a cookie so that returning user's are pinned to their theme of choice.